X-rays are electromagnetic radiation of a very short wavelength, in the range of 0.05 angstom units to a few hundred angstrom units. Like other forms of elctromagnetic radiation, they have the same speed in vacuo, "c" (3x10⁸ ms^-1), and show properties associated with waves such as interference, diffraction and polarisation.

X-rays were discovered by Wilhelm Conrad Röntgen, in 1895. Whilst studying cathode rays produced by electical discharges in low pressure gases he noticed a strange event. He noticed that a surface coated in barium platinocyanide placed outside the discharge tube would emit light (fluoresce). Even when he shielded the surface from the UV and visble light from the discharge tube the barium platinocyanide continued to fluoresce. He came to the conclusion there was invisble radaition being generated and named these new rays as X-rays to indicate the unkown nature of them.

X-rays have many applications. One of the earliest applications was in medicine and they are still used today in diagnosis of fractures, foreign objects in the body, dental cavities and the therapy of malignant tumours. In industry x-rays are used to test materials - by measuring the absorbance of x-rays by a known material the thickness can be calculated. In chemistry, x-rays are used in studying the structures of crystals - x-ray crystallography.